A. Technical Field
The present invention relates to a process for producing a water-absorbing agent, and a water-absorbing agent.
B. Background Art
In recent years, water-absorbent resins (water-absorbing agents) are widely used as one of materials constituted of absorbent articles, such as disposable diapers, sanitary napkins, and incontinent pads, for the purpose of causing the water-absorbent resins absorb body fluids.
The above-mentioned water-absorbent resins (also called as higher-water-absorbing resins or water-absorbent polymers) are, for example, disclosed in Japanese Industrial Standard (JIS) K7223-1996, and was already well known.
Known examples of the above water-absorbent resins are as follows: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, and their crosslinked products; and crosslinked polymers of cationic monomers.
Examples of the properties which the above water-absorbent resins should have are as follows: upon contact with aqueous liquids such as body fluids, excellent water absorption amount or speed, the liquid permeability, the gel strength of the swollen gel, the suction power to suck up water from a base material containing aqueous liquids from the past. Then, when the water-absorbent resins have a plurality of these properties together from the past, and used as absorbent articles, such as disposable diapers, and sanitary napkins, various water-absorbent resins (water-absorbing agents) displaying excellent performance (absorbing property) are proposed.
For example, as to a method for improving the water-absorption properties of the water-absorbent resin, such as absorption capacity without load and absorption capacity under a load in good balance, there is a known art in which the surface neighborhood of the water-absorbent resin is crosslinked, and various methods have ever been proposed.
For example, there are known methods in which the following materials are used: polyhydric alcohols (JP-A-180233/1983 and JP-A-016903/1986); polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds (JP-A-189103/1984); glyoxal (JP-A-117393/1977); polyvalent metals (JP-A-136588/1976, JP-A-257235/1986 and JP-A-007745/1987); silane coupling agents (JP-A-61-211305, JP-A-61-252212, and JP-A-61-264006); alkylene carbonates (DE 4020780); oxazolidinones (WO 99/43720); oxazines (WO 00/311539); and oxazoline compounds (JP-A-197818/2000).
In addition, as to water-absorbent articles such as diapers, the amount of water-absorbent resins as used tends to increase. Therefore, when various water-absorbent articles such as diapers are produced by use of water-absorbent resins, it is necessary to include a plenty of highly hygroscopic water-absorbent resin powders in fiber base materials, and there is a problem such that the water-absorbent articles cannot be produced stably because the water-absorbent resin powders each other cause blocking in a hopper or in the middle of a production line, or are attached to apparatuses depending upon its working circumstances, or climate conditions (especially, in an area having high temperature and high humidity). That is to say, the water-absorbent resins are often produced as powders. Therefore, the fluidity of the powders, especially the fluidity under a hygroscopic atmosphere is important for a practical use.
There are known methods in order to solve the fluidity under such a hygroscopic atmosphere: compositions obtained by blending water-absorbent resin powders with a hydrophobic silica fine particles in a specific ratio (JP-B-17542/1986); compositions obtained by blending water-absorbent resin powders and inorganic particles, such as water-containing silicon dioxide, water-containing aluminum dioxide, and water-containing titanium dioxide (JP-A-80459/1984); methods which involve treating water-absorbent resins with specific cationic surfactants, and thereafter blending inorganic substances or high-melting-point organic compounds (JP-A-69854/1986); methods which involve blending water-absorbent resin powders with stearic acid and inorganic particles (JP-A-105064/1988); and methods which involve treating water-absorbent resins with specific silicone surfactants (JP-A-136966/1997).
However, there is a problem of cost of the additives or safety of powder dusts in conventional arts, and further there are many problems such that: the production process for water-absorbent resins are complicated, or the properties of the water-absorbent resins are decreased. Therefore, water-absorbent resins (water-absorbing agents), obtained by modifying the balance between the absorption capacity without load and the absorption capacity under a load and the fluidity under a hygroscopic atmosphere, were insufficient.
Furthermore, there is a problem of safety of the crosslinking agent as used. When the crosslinking agent is a compound having a reactive functional group such as an epoxy group in general, the reaction can be carried out at low temperature. However, there are problems such that: it is necessary to strictly control the residual amount of the crosslinking agent because the crosslinking agent itself is irritant to skins; and complicated procedures are necessary in the processes to reduce the residual amount. In addition, safe crosslinking agents such as polyhydric alcohols have low reactivity. Therefore, they require a high temperature or long time in the crosslinking reaction. There is a possibility that the water-absorbent resins are colored or deteriorated due to heating.
A. Objects of the Invention
The present invention was completed in accordance with the above-mentioned conventional problems. That is to say, an object of the present invention is to provide: a process for producing a water-absorbing agent at low temperature in a short time, wherein the water-absorbing agent has excellent balance between the absorption capacity without load and the absorption capacity under a load, high fluidity under a hygroscopic atmosphere, and excellent safety. In addition, another object of the present invention is to provide: a novel water-absorbing agent, and a novel crosslinking agent particularly suitable for crosslinking water-absorbent resins.
B. Disclosure of the Invention
The present inventors diligently studied a water-absorbing agent having excellent absorbing properties and fluidity under a hygroscopic atmosphere. As a result, they found that the above problems were solved by using a specific compound as a crosslinking agent of the water-absorbent resin.
That is to say, a process for producing a water-absorbing agent, according to the present invention, is characterized by comprising the step of blending an oxetane compound and/or an imidazolidinone compound with a water-absorbent resin.
In addition, the present invention is characterized in that the oxetane compound is represented by the following structural formula (1) in the above production process for a water-absorbing agent,
wherein the structural formula (1) is: 
where: R1, R2, R3, R4, R5, and R6 each independently represent a hydrogen atom, a hydroxyl group, an alkyl group, an alkenyl group, an aromatic group, a halogen group, a substituted alkyl group, a substituted alkenyl group, or a substituted aromatic group.
In addition, the present invention is characterized in that the imidazolidinone compound is represented by the following structural formula (5) in the above production process for a water-absorbing agent,
wherein the structural formula (5) is: 
where: R1, R2, R3, R4, R5, and R6 each independently represent a hydrogen atom, a hydroxyl group, an alkyl group, an alkenyl group, an aromatic group, a halogen group, a substituted alkyl group, a substituted alkenyl group, or a substituted aromatic group.
An absorbing agent, according to the present invention, comprises: an oxetane compound and/or an imidazolidinone compound; and a water-absorbent resin.
In addition, an absorbing agent, according to the present invention, comprises a water-absorbent resin crosslinked with an oxetane compound and/or an imidazolidinone compound.
In addition, an absorbent article, according to the present invention, comprises the above-mentioned water-absorbing agent.
In addition, the present invention relates to a crosslinking agent for water-absorbent resins, which comprises an oxetane compound having at least one oxetane skeleton per a molecule.
In addition, the crosslinking agent for water-absorbent resins is characterized by having a structure represented by the following general formula (1),
wherein the structural formula (1) is: 
where: R1, R2, R3, R4, R5, and R6 each independently represent a hydrogen atom, a hydroxyl group, an alkyl group, an alkenyl group, an aromatic group, a halogen group, a substituted alkyl group, a substituted alkenyl group, or a substituted aromatic group.
In addition, the present invention relates to a crosslinking agent for water-absorbent resins, which comprises an imidazolidinone compound having at least one oxetane skeleton per a molecule.
In addition, the crosslinking agent for water-absorbent resins is characterized by having a structure represented by the following general formula (5),
wherein the structural formula (5) is: 
where: R1, R2, R3, R4, R5, and R6 each independently represent a hydrogen atom, a hydroxyl group, an alkyl group, an alkenyl group, an aromatic group, a halogen group, a substituted alkyl group, a substituted alkenyl group, or a substituted aromatic group.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.